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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

Posted on 26 February 2013 by gws

A new paper by Hemming et al. presents physical, best case limits on the (opposing) effects of warming and CO2 on the “greening” of the biosphere. Using the Hadley Center’s general circulation model in connection with an interactive plant response, they show that a doubling of atmospheric CO2 in the model under no other limitations increases global terrestrial net primary productivity (NPP - overall plant growth) on average by 57%, spatially dominant in the tropics. While warming alone decreases NPP, the physiological effect of higher CO2, on average, more than compensates for the losses globally. The new study uses an innovative “perturbed physics ensemble” (PPE), similar to ensemble weather forecasting, to study how quantitatively and spatially relevant the results are. However, as critically important nutrient limitations and potentially important biome shifts were excluded from this first study, the results are of limited relevance as they represent only a best case scenario.

The latter process, known as a major part of the the terrestrial carbon cycle, consists of many individual steps and sub-processes of various complexity, which makes the field of terrestrial carbon cycle research one of the most heavily researched fields of the geosciences. It comes, therefore, maybe as no great surprise that the general public has only a limited understanding of the importance of carbon cycling, and that contrarians seeking to confuse and sow doubt among the public frequently cherry-pick only one aspect of the carbon cycle, the “carbon fertilization effect”.

CO2 is needed for plant growth, thus more CO2 will lead to more plant growth, and hence to a “greening” of the Earth. Ergo, more CO2 is good for Earth.

There are multiple reasons why such a gross simplification of the role of atmospheric CO2 in the terrestrial carbon cycle is short-sighted and strongly misleading. First of all, there is no real-world situation, in which only CO2 increases, all else remaining equal. And second, those other effects of increased atmospheric CO2, namely increased temperatures and altered moisture regimes, generally have adverse effects on growth.

SkS has addressed the meme at various points, prominently here, and recently here. However, the importance of the terrestrial biosphere’s reaction and any possible feedbacks to the atmospheric CO2 increase is part of much ongoing research. Rightfully so, because humanity critically depends on the terrestrial biosphere, such as for food production and clean water just to name two obvious aspects. How our biosphere reacts to warming and higher atmospheric CO2 will ultimately define how catastrophic we can expect our global CO2-experiment with the climate to be. So wouldn’t it be good to know more about how much “greening” to expect and what it means?

NPP and CO2 fertilization

All land plants carry out green leaf photosynthesis, the process of acquiring and converting atmospheric CO2 initially into simple sugars and from there into all other carbon-containing plant matter. Next time you look at that bush or tree in your yard, realize that about 50% of its dry mass is carbon, once part of the atmosphere in the form of CO2. Over the course of each growing season, i.e. when ambient temperatures allow for active photosynthesis, plants accumulate carbon out of the atmosphere and store it, they grow. The net growth over a year is called the Net Primary Productivity, or NPP. Globally, it is estimated to have been around 60 Petagram (Pg, 1015 g) carbon in the 1990s, balanced by an equal amount or carbon returned to the atmosphere via (heterotrophic) respiration. Much of that respiration is occurring from degrading plant material in soils, meaning respired carbon entering the atmosphere is “older” than the newly stored carbon, and the biosphere (plants and soils) itself acts as an intermediate storage reservoir for carbon cycling in and out of the atmosphere. But as humans also appropriate a large amount of global annual NPP for food and products, some carbon returns faster.

Because the first step of photosynthesis is the diffusion and (biological) absorption of atmospheric CO2 into a leaf, increasing CO2 surrounding that leaf will initially speed up the photosynthetic uptake. In other words, the initial photosynthesis steps can be treated like a first-order process, in which the rate of uptake of CO2 is a function of CO2 abundance itself. Thus the fertilizer analogy: You feed it more, it grows more. However, and this is the first major culprit, this dependence of uptake on abundance is not linear but drops rapidly with increasing CO2, ultimately flatlining above 1000 ppm CO2. The second major culprit, or better say group of culprits, is that atmospheric CO2 is not the only factor affecting photosynthesis and plant growth. Plants need many nutrients, and water, for optimal growth. Unless these nutrients, such as nitrogen or phosphorous acquired out of the soil, are provided in amounts increasing proportionally to the supply of CO2, no continued fertilization effect of CO2 can be sustained.

Calculating NPP changes

It is impracticable to measure carbon cycling rates on a global basis accurately. While the global atmospheric CO2 monitoring network allows us to estimate how much atmospheric CO2 is removed by the terrestrial biosphere annually, that amount only represents the difference between NPP and respiration, and does not show a correlation with atmospheric CO2 (cf. discussion by the Global Carbon Project).

Because there are so many factors affecting annual terrestrial NPP, atmospheric CO2, air temperature, nutrient and water availability, season length, seasonal temperature development, air pollution and other anthropogenic interferences to name a few, it is very complicated to provide a reliable estimate from knowledge of all relevant processes and their dependencies on physical and biological parameters. Nevertheless, the last decade has shown a flurry of global modeling efforts that combined climate modeling with terrestrial biosphere modeling. The present paper makes an important step forward as it tries to address why past findings have at times identified a larger or smaller fertilization effect, and sometimes even net carbon losses from the biosphere.

Similar to ensemble forecasting in weather research, where a base set of input parameters to the forecast is varied within physically reasonable boundaries to create an ensemble of forecasts that most often describes reality more accurately then each single forecast itself, the researchers used a base set of physical variables in their climate model to create a series of globally gridded ensemble (equilibrium) climate and associated NPP projections, with or without allowing for the physiological effect of CO2 on photosynthesis. The latter allowed them to distinguish between the effects of warming itself on NPP (adverse) and that of warming and atmospheric CO2 on NPP (favorable). The perturbed physics ensemble, PPE, was used to gauge how robust the NPP results were among the two drivers, climate and CO2 in pre- and post-industrial [CO2] worlds, meaning whether the effect, if any, could have arisen through random uncertainties in the climate model parameters themselves represented by the PPE spread.

Mixed news for the tropics

Most of global NPP is accumulated in the tropics, Figure 1, a result consistent with all prior work on global NPP. The tropics are “lush” and became more so under doubled CO2 in the model. While warming caused NPP losses throughout the tropics in the model (driven by temperature effects on autotrophic respiration), the physiological effect of CO2 dominated the net, Figure 2.

However, the standard deviation of the PPE was also high in several regions of the tropics, Figure 3, prompting the authors to state

“In these locations, NPP is particularly sensitive to the specific choice of model parameterisations, such that even the sign of the NPP response could change according to the parameterisations adopted.”

So while there appears to be a robust signal that tropical NPP will increase as a result of increasing atmospheric CO2, the spatial forecast is mixed.

Less good news in mid- and high latitudes

Outside the tropics, in the major food production regions of the northern hemisphere, the model's verdict is less encouraging. The model’s NPP increases were of similar magnitude than the PPE standard deviation, Figure 3, and the authors highlighted larger uncertainties of warming than CO2 on expected NPP changes.

“… these results indicate that the direction of the global average NPP response to doubled [CO2] is likely to be positive, regardless of the values of the model parameterisations perturbed in this study.”

Insofar, one can hope that, and there is broad scientific consensus, that there will indeed be a small amount of “greening” from increased atmospheric [CO2]. However, stressed at several points in the manuscript, the above scenario is for a situation where the basic composition and structure of the global terrestrial biosphere does not change under doubled [CO2] and where nutrient limitations do not matter. As the primary nutrients nitrogen and phosphorous are key limiting factors in many ecosystems, particularly in the tropics, we can expect that real-world NPP increases as a result of doubling [CO2] will be much lower than the ensemble average of 57% the authors calculated. And we expect that similar studies addressing this issue will be published in the coming years.

An additional word of caution

Model NPP in GCMs is generally an equilibrium value. It does not tell us anything about transient carbon fluxes, such as those triggered by fires, floods, heat waves or droughts, extreme events that are expected to increase in frequency as warming progresses. As the (devasting) effects of the US drought since 2011 are becoming more and more apparent, it is clear that even potentially large increases in crop yields cannot be sustained in an increasingly extreme weather world. Especially heat waves, e.g. the Europe-wide heat wave in 2003, can cause widespread plant mortality and crop yield drops that are expected to cause regular food security issues in the second half of the century.

Summary

Doubling atmospheric CO2 is likely going to cause some "greening" of the terrestrial biosphere globally under equilibrium conditions, especially if factors such as water and nutrient availability do not become limiting

Likely, much of the greening will manifest itself in the tropics, not in mid-latitudes; warming negates much of the CO2 effect

Limiting growth factors are common throughout the terrestrial biosphere, thus the calculated "CO2-only" effects represent a best case scenario

Equilibrium conditions are a model construct not accounting for transient developments, particularly short-term weather extremes with the potential to eliminate long-term gains

Deborah Hemming, Richard Betts, Matthew Collins:

Sensitivity and uncertainty of modelled terrestrial net primary productivity to doubled CO2 and associated climate change for a relatively large perturbed physics ensemble,

Comments

Interesting. Did the original paper discuss the findings of Nitrogen constraints on terrestrial carbon uptake: Implications for the global carbon-climate feedback (Link) from back in 2010? Wang and Houlton found that the increased temperatures were likely to seriously reduce fixed nitrogen, especially in the tropics, so that the overall global availablity of fixed nitrogen was likely to drop.

On the same day that gws posts this excellent article about the relationship between future food production and CO2 concentrations in the atmosphere, Justin Gillis posts Feeding Ourselves on a Warming Planet on the New York Times' Green Blog.

This is good news. However, as stated "atmospheric CO2 is not the only factor affecting photosynthesis and plant growth". So far, we have massive pine beetle die-back in the Western and NorthWestern U.S. We have 'once-in-500-year' droughts in the Amazon occuring twice in ten years. We have a Russian heatwave that spiked global wheat prices. We're in the third year of drought in the U.S. breadbasket. Live coral acreage is tanking. Clearly, something that is currently CO2-limited is going to be very happy in the near future. What does that mean for us? It may be a little like celebrating flu outbreaks on behalf of the viruses.

Truly, "atmospheric CO2 is not the only factor affecting photosynthesis and plant growth".

We know that the increasing CO2 is coming from burning fossil fuels. Another consequence of burning either fossil fuels or biofuels is the increase of NOx and Ozone in the troposphere - which has really serious impacts on all growing plants, from smallest to largest.

I don't think this study means much at all. The stuff that is supposed to be getting greener - is also going to be getting deader!

Two points worth remembering are that plants have evolved: 1. In conditions of climate stability and 2. at locations best suited to their survival.

Increased CO2 concentration promotes plant growth but it also promotes climate change characterized by extreme heat, drought and precipitation events which plants, particularly food crops, can not handle.Further, the frequency of those events is expected to increase with regional warming, making the best locations for plant growth at present unsuitable for their growth in the future.Food production is particularly vulnerable to a warmer more volatile climate because increased heat diminishes crop yield.

There is a tendency among some commentators to overlook the fact that elevated CO2 promotes both crop and weed growth and that the latter competes with crop plants for limited water and soil nutrients, both of which are impacted by rising temperature.

It is easy to assert that “we can adapt” to changing climate conditions by developing heat tolerant food crop varieties but quite another thing achieving it.New varieties must also be able to produce increased yield while using less nutrients to do so and be able to simultaneously tolerate drought, higher rainfall and increased resistance to insects.Bit of a tall order!

I think this is a typo: ha = 10 000m2, therefore it should be 10 000kg or 10 metric tons.

To put the fertilisation effect in a proper context, it would be interesting to know how much of the increased fertilisation effect calculated by this study is currently happening with 40% CO2 increase since preindustrial. We know the fraction of anthropogenic C imbalance uptaken by ocean invasion and by the NH terestrial biosphere (from emissions - Mauna Loa measures and the isotopic footprint of C fluxes), so I think the direct comparison of the existing perturbation of AT carbon cycle and the carbon cycle predicted by this study is possible. Does anyone know of such comparison and can cite some numbers?

Chriskoz - it's still a controversial topic. Clearly land-based plants (mostly trees) are drawing down a sizeable chunk of human CO2 emissions, but is this due to forest re-growth in the tropics, former Soviet Union countries, and China, or is this the fabled fertilization effect?

angliss @1: the Wang paper you mention was not cited, however, the authors are well aware of nutrient limitations and have emphasized that their model does not account for them. Future work is supposed to take this additional step.

keenon350 @4: check out http://aspenface.mtu.edu/ ; ozone is indeed a major culprit, and we tend to underestimate its impact because we are not aware of how much more NPP we could actually get with low to very ozone abundances (<10 ppb), because those do not exist any more in the real world (background now typically is already >30 ppb).

chriskoz @6: numbers in caption fixed. In the paper they actually calculated a lower than contemporary NPP for pre-industrial [CO2], which they attributed "in part" to the lower [CO2]. But to address your question: I would not call the science on this "controversial", but rather "complex". At a particular study site where in-depth research allows studying local carbon cycling as a function of local climate, nutrient availability, and other factors, one may tease out a CO2-fertilization effect, and FACE projects were designed to particularly address the CO2 effect. But in a global model where local conditions are blended by averaging over time, space, and ecosystem type, I think one cannot hope to resolve the numerous drivers such that an accurate picture emerges everywhere. You may get it right for a mature boreal forest but not the African savanna or vice versa. However, the models are of course informed by results from the flux network and other measurements, so they will become more accurate over time, also because more ecosystem flux measurements are now in place in a larger variety of ecosystems than 10-20 years ago. Nevertheless, the models are mostly for guidance of what to expect under different scenarios.

To recap my understanding, together with some numbers. Looking at this Carbon Cycle picture and figures 1 (NPP at preindustrial CO2 280ppm) and 2 (dNPP per doubled CO2 to 560ppm, let's call it "NPP sensitivity" or dNPP) above, I deduce:

- NPP 60Pg (or 60Gt) per 150Mkm2 (land surface area) means 0.4kg/m2 and that is the average value on Figure 1

- It's hard to eyeball the average NPP sensitivity from Figure 2 but it looks as "green" as Figure 1, then in the order of 0.2-0.4kg/m2, therefore average NPP sensitivity could result in a staggering doubling of 57pG NPP flux shown on carbon cycle and drawing that +204Pg from the atmosphere pretty quickly. That conclusion sounds incredible, prodived that fertilisation effect has hard limits and levels out in most autophytic species. I would not expect that dNPP could have such potential (in the ideal conditions of abundance of water & other nutrients). Perhaps I read Figure 2 incorrectly or my calculations are wrong.

- How much of that red 2.4Pg "Land sink" flux shown on Carbon Cycle picture is due to dNPP, remians highly uncertain.

the actual average increase was 57%, given in the post; that is much less than your "eyeball" estimate, but also varying a lot between biomes. "Eyeball" estimates are often wrong when the underlying data is not normally distributed. I thus dislike, and did not include in the post, that the authors gave an average 0.293 kg m-2 y-1 (range 0-2.12 kg m-2 y-1!) in the paper. That average may be realized in one biome, and is not representative in an ecological sense. In any case, it is applicable mathematically to an area of 123 M km2 (not all land area is vegetated). High increases occurred in the model where NPP was high in the first place, particularly the tropics.

I guess one could define a "NPP sensitivity" similarly to climate sensitivity in order to have a common reference point (such as for comparing model outputs). But, similar to climate sensitivity, it remains a function of actual [CO2] level, i.e. incremental warming changes for each subsequent doubling and incremental NPP increase change with each doubling. If you follow the link in the post, you will find that between 280 and 560 ppm, the slope of the curve is still relatively large, but for any additional increases it will drop rapidly, more rapidly than the parallel warming effect. Meaning, even in the hypothetical case of a biosphere reacting solely to [CO2] and uptake of most of anthropogenic CO2, this counter-effect to warming could not be maintained as [CO2] increases.

the current estimate for the land sink, 2.6±0.8 Pg C y-1, is calculated as the residual from better known fluxes, i.e. anthropogenic emissions, fraction remaining in the atmosphere, uptake by the ocean, and net land use change fluxes. The latter is positive, meaning gross land uptake is actually larger than 2.6 Pg (follow the Global Carbon Project link). The new paper (still in review in Earth Syst. Sci. Data) by Le Quéré et al. that is linked from the presentation shows that a current set of Dynamic Global Vegetation models (DGVMs) does a decent job in calculating magnitude and interannual variability of the land carbon sink. The models include the fertilization effect as well as deforestation, afforestation, regrowth, and climate variability. Some include nitrogen dynamics. In general, these models are more sophisticated than the one discussed in the post (which focussed on varying climate model parameters into one biosphere model), but they were all run with the same climatic inputs. You would have to turn off all other relevant effects for increasing NPP (regrowth, afforestation, nitrogen fertilization) in order to tease out the CO2 fertilization effect itself. Not yet done. Hence my announcement in the post ... wait for it to come soon.